There is no admission that the background art disclosed in this section constitutes prior art.
To be able to use a particle beam, such as an electron beam, by way of example and not by way of limitation, to create a pattern on a substrate, it is important to know the profile of the beam. The profile of the beam refers to the intensity of the beam as a function of position/location on the beam. Knowledge of the profile of the beam is innately valuable, because it shows how the particles, electrons for example, are being put down on the substrate, which determines how the pattern is formed. Since formation of the pattern on the substrate is carried out by making one little shape, and then another right next to it, and another little shape next to that, understanding at the very edges of the shapes how those shapes combine is important.
For example, when a the pattern being formed is an image in a photoresist, whether for direct production of a semiconductor device, or in the fabrication of a photomask/resist, the pattern is being created “blind”, where the actual pattern being created is not visible. Knowledge of the profile of the beam enables the technologist to be able to predict the pattern image which is being created in the photoresist. In fact, knowledge of the profile of the beam enables the technologist to “tune” the beam profile in order to obtain a desired pattern image in the photoresist. The general shape of the beam may be a rectangle, a triangle, or a square, for example, and the beam profile will be different for each general shape.
Typically, a particle beam profile such as an e-beam profile has been determined by scanning the beam relative to edges in two orthogonal directions and measuring the beam current that is not obscured by the grid with a detector. Most simply, the edge is held stationary, and the particle beam is scanned (position is changed via deflection as a function of time) over the edge while the number of particles (e.g. current) that pass the grid striking a detector is monitored as a function of time. Because both the position of the particle beam and the current are known as related to time, the current is also known as a function of position. The current striking the detector is the integral of the particle-beam current density that is not obscured by the edge. To re-obtain the particle-beam current density profile, software may be used to take the derivative of the current versus position function.
One of skill in the field of particle beams will recognize that there are several shortcomings in the simple method of measuring a beam current-density profile; particularly if the beam is blanked (rapidly turned on and off) or scanned during normal use, as these actions may distort the dose deposited on the target as compared to the beam profile. In this sense, the dose is the time integral of the particle-beam current-density profile as it strikes the target. For example, instead of continuously scanning the beam over the edge, the beam may be stepped over the edge. In stepping, the beam is moved in small increments and then held steady while the current is measured in each position for a longer time. The longer time period at each beam position relative to the edge allows for a more accurate measurement of the current at each position. Typically 100 to 1000 unique positions would be measured, and particle beams may range in extent from 25 nanometers to 2000 nanometers. The exact relationship between the number of unique positions and the size of the particle beam will depend on the spatial resolution required by the application and the time allowed to take the measurement. For maximum resolution, the distance between unique steps must be less than ½ the width of the point spread function (PSF) of the metrology array edges, which, for the invention described herein would be less than about 1.5 nanometers(nm).
Additional complexity may be added to the measurement to obtain more accurate information. For example, the particle beam may be blanked on and off repeatedly at each unique measurement position to account for blanking affects. If the particle beam is scanned during normal operation, the blanking may be combined with scanning the beam where the beam is unblanked at the position of the scan corresponding to the unique measurement position.
A metrology array is used to measure beam profile. The metrology array most commonly used has been a square lattice. The openings in the array are typically such that the ratio of opening to bar is about 1:1. A limiting factor of this design is the heat conduction from along the bar, due to interaction of the beam with the bar. The limited availability of heat conduction changes the temperature at the measurement point, and thermal expansion at the measurement point causes the grid to drift. When the grid drifts with respect to the particle beam, the measurement changes, making the measurements inaccurate.
A critical figure of merit for a metrology array is the width of the point-spread function (PSF). The PSF is the effect of convolution between the metrology array and the particle beam. To convincingly measure the profile of an electron beam, without relying on deconvolving the metrology array, for example, the PSF of the array should be at least 3 times smaller than the intrinsic blur of the electron beam. The blur of the electron beam as used herein means the width of the profile edges. For example, one metric of the blur is the Y direction or X direction distance between 20% and 80% of the maximum current density on a plot of the kind shown in FIGS. 4-6 of the present disclosure. To measure the profile of an electron with intrinsic blur of 10 nm, the PSF of the metrology array should be in the range of 3 nm. The current state of the art for a metrology array is approximately 30 nm, although isolated edges with the required PSF exist.
In an effort to reduce the PSF of the metrology array, efforts have been made to increase the effective opaqueness of the material on the surface of the array. The scattering of electrons passing through the array material is analogous to the opaqueness of the material. Since the scattering probability of a material is proportional to Z2, high-Z materials are often deposited on silicon metrology arrays after fabrication of the array. This not only requires additional processing of the array, but also roughens the edges of apertures in the array, further increasing the PSF of the aperture.
Another important factor in determination of the PSF of the metrology array is the kind of edge present at the edges of the apertures in the array. Silicon metrology arrays typically are created using an etch process that creates sidewalls which are at about a 55 degree angle with respect to a horizontal surface beneath the sidewalls. This angle causes an increase in the PSF due to the limited material thickness near the edge of the aperture, making opaqueness a function of distance from the edge. To emulate a step function in opaqueness of the aperture, those skilled in the art have created high-angle edges (knife edges) by cleaving a crystal of indium phosphide, for example. This method of creating edges creates only a single edge and is not reliable in terms of a manufacturable, reliable means of edge creation.
There is currently a need in the industry to reduce the PSF of a metrology array, so that better resolution of a particle beam profile measurement can be achieved.